A series of phosphinic acid derivatives will be synthesized and tested as irreversible inhibitors of the three forms of purified thiolase (EC 2.3.1.9 and EC 2.3.1.16). These inhibitors are designed to mimic the unstable tetrahedral intermediate formed during the enzyme catalyzed Claisen condensation of two molecules of acetyl-CoA. Based on analogy with the past successes in the protease field, these inhibitors should be either rapid, tight binding transition state analogs or slow, tight binding inhibitors. In either case, the specificity of these inhibitors for thiolase will be considerably greater than the inhibitors currently in use (ie alpha-halo acetylCoA). The inhibitors will be tested by steady state kinetic analysis of the three thiolases functioning in (1) fatty acid beta-oxidation, (2) ketone body metabolism, and (3) cholesterol biosynthesis. Specific, stable, potent inhibitors of these enzymes can be used to study in vivo cholesterol biosynthesis under conditions where the first step of the biosynthetic pathway is inhibited.